How I Learned to Avoid Magical Dependency Injection And Love Plain Java

A short story about the complexity of magical frameworks and dependency injection with a happy ending, featuring Resteasy, CDI, and JBoss.

Once upon time, I have created a JAX-RS webservice that needed to supply data to a user's session. I wanted to be fancy and thus created a @Singleton class for the exchange of information between the two (since only a user request serving code can legally access her session, a global data exchange is needed). However sharing the singleton between the REST service and JSF handler wasn't so easy:

Originally, the singleton was generic: OneTimeMailbox<T> - but this is not supported by CDI so I had to create a derived class (annotated with @Named @Singleton)

While everything worked in my Arquillian test, at runtime I got NullPointerException because the @Inject-ed mailbox was null in the service, for reasons unclear. According to the internets, CDI and JAX-RS do not blend well unless you use ugly tricks such as annotating your service with @RequestScoped (didn't help me) or use JBoss' resteasy-cdi module.

Finally I got fed up by all the complexity standing in my way and reverted to plain old Java singleton (OneTimeMailbox.getInstance()) while making testing possible with multiple instances by having a setter an alternative constructor taking the mailbox on each class using it (the service and JSF bean) (using a constructor might be even better).